Intake charge flow has a large impact on the performance of gaseous fuelled internal combustion engines. The mixing of air and possibly exhaust gases with gaseous fuel influences the quality of combustion inside the combustion chamber. The charge motion inside the combustion chamber during the intake stroke and later during the compression stroke determines the level and quality of mixing of the gaseous fuel. In some parts of the engine map a homogenous air-fuel charge may be desired, in other parts of the engine map a stratified fuel charge near an ignition device improves engine performance, and in still other parts of the engine map a locally rich and globally lean air-fuel mixture yields better performance. The production of high turbulence is an important factor for stabilizing the ignition process and for fast propagation of flame fronts (flame speed), especially in the case of lean-burn combustion. Two techniques for creating charge motion within cylinders are known as tumble motion and swirl motion. Tumble motion and swirl motion can be characterized by a dimensionless parameter employed to quantify rotational and angular motion inside the cylinder, which are known as tumble ratio and swirl ratio respectively. Both these values are calculated as the effective angular speed of in-cylinder air motion divided by the engine speed.
It is known to use tumble motion for a direct injection light duty gasoline engine employing fuel stratification around an ignition device. In tumble motion, which is also referred to as vertical swirl or barrel swirl, the rotation axis of the intake charge in the cylinder is orthogonal to the cylinder axis. In the context of this application a light duty engine is one having a cylinder bore diameter less than 90 millimeters (mm). Fuel stratification is an effective technique to extend lean burn limits in spark ignition engines and therefore gives an increased fuel economy, and exhaust emissions are reduced compared to previous gasoline light duty engines. Tumble motion can be effective in creating high levels of near-wall flow velocities even relatively late in the compression stroke, which can promote evaporation of a fuel wall film that is formed by an impinging fuel spray.
U.S. Pat. No. 5,553,580, issued Sep. 10, 1996 to Ganoung, discloses a high squish area barrel-stratified combustion chamber for gasoline engines employed to reduce brake specific fuel consumption for light duty engines. Two intake valves are in fluid communication with respective intake passages that are configured as tumble ports. A barrel-stratified charge is created in a cylinder by introducing gasoline into one of these intake passages such that a stratified barrel swirl forms in the vicinity of an asymmetrically located spark plug. The barrel swirl does not enhance burn rate, but rather promotes stratification of air-fuel charge in the cylinder at the time of ignition of the spark plug. A large squish area provides a fast burn rate by enhancing turbulence intensity during combustion.
It is known to use swirl motion for a diesel-cycle (compression ignition) heavy duty engine. In swirl motion the rotation axis of the intake charge in the cylinder is the cylinder axis. In the context of this application a heavy duty engine is one having a cylinder bore diameter greater than 120 millimeters (mm). Swirl motion has been shown to reduce particulate matter (PM) emissions from the engine. The trend for compression ignition engines is to employ higher injection pressures, which for liquid fuels improves droplet break-up, and for both liquid and gaseous fuels higher injection pressures improve air/fuel mixing in the spray and increases turbulent intensity in the combustion chamber. This is important, especially during transient conditions when the combustion system must handle lower air-fuel ratios conditions without producing high PM emissions. When swirl motion is employed the effects of PM production can be reduced under some transient conditions, even when high injection pressures are used. Converting an engine designed for swirl motion to tumble motion requires a different orientation for the intake passages and this requires a different cylinder head. The need for a new cylinder head is a deterrent to experimentation with this technology for medium duty engines and larger engines because diesel engines are already considered to be the most efficient internal combustion engines.
A goal of engine design is to downsize the displacement volume of cylinders without substantially losing performance (horsepower and torque). With increased fuelling costs and street congestion, vehicle operators are demanding more compact vehicles that provide the same overall performance as large vehicles but with improved fuel economy. Alternative gaseous fuels are increasingly finding new applications in automotive market segments dominated by gasoline and diesel fuelled engines in many jurisdictions. In light duty applications port injected natural gas engines have a long history in the aftermarket segment, and more recently OEM versions of these vehicles are being introduced. In heavy duty applications high pressure direct injection (HPDI) engine systems match the performance of diesel fuelled engines and with improved fuel economy compared to port injection natural gas engines.
There is a need for gaseous fuelled engines with comparable performance to larger engines but with improved fuel economy especially for engines designed at least for medium duty service.